AGC Circuit

The basic idea for this circuit came from the "Progressive Communications Receiver", by Wes Hayward, W7ZOI & John Lawson, K5IRK, QST, November 1981, Page 17, Fig. 8.

The transistor can be almost any 2N2222 or 2N3904 type or substitute. The 1N270 is a germanium diode, and any germanium diode will work. If all you have are 1N914's, measure the reverse voltage with a DVM (most of them have this function - the diode symbol), and use the ones with the smallest reverse voltage reading. Your penalty is a less sensitive S-meter reading, but by an amount that you would hardly notice. The 1N270's run about .3 volt whereas the 1N914's run .5 to .7 volt.

If you are going to use this circuit in another receiver, be aware that as the IF frequency increases, the less voltage or current is developed across the diode and capacitor (rectifier and filter) combination. It's harder to get AGC action the higher you go in frequency.

One of the reasons for the 455kHz IF strip is so you can achieve a great AGC. At 455kHz, plenty of voltage is developed to have a fast acting, active AGC. According to Rohde, the best AGC is when the S-Meter is barely activated from the band noise level. That is achieved with this IF strip and AGC circuit.

The two diodes connected to the 10K resistors, one going to ground and the other to 12 Volts, keep the voltage at both the collector of the transistor and Gate 2 of the MOSFETs at 1/2 the supply voltage, or approximately 6 volts. Both circuits are fed with 1 megohm resistors. The 1 megohm resistors and the 10 mfd electrolytic (or tantalum capacitor) set the response times.